Understanding Society is a large panel survey, involving 100,000 individuals living in households in Great Britain. In 2012, for the first time, a sequential mixed mode approach was piloted, involving first Web and then face-to-face data collection for non-responders to the Web. The questionnaire was designed to collect equivalent data in both modes, using a single instrument. The pilot was undertaken with members of Understanding Society’s Innovation Panel (IP), who may have taken part in up to four previous waves of data collection, all involving face-to-face interviews. Panel members were randomly allocated to either a mixed mode or single mode data collection group, the latter involving only a face-to-face interview. This was done, in part, to assess the impact of adopting a sequential mixed mode design on response rates. While the Web response was higher than expected, a statistically significant difference in response rates between the two groups (mixed mode and single mode) was found, with the response rate for individuals being lower among the mixed mode group. Moreover fewer interviews were achieved with all members of the household in the mixed mode group than in the single mode group. To understand more about why these differences occurred we undertook qualitative follow up interviews with members of the mixed mode group to answer two specific questions.  Why were respondents in the mixed mode sample group, who did not respond by Web, less likely to participate in a face-to-face interview than those in the single mode group?  Why were members of households where one other person had completed by Web less willing to take part in the survey, in either mode?